* First summit between two leaders
* They are to meet in Russian Pacific port
* Nuclear standoff on agenda: Kremlin aide
* North Korean leader expected to arrive by train (Updates with preparations, background)
By Vladimir Soldatkin and Maria Vasilyeva
MOSCOW/VLADIVOSTOK, Russia April 23 (Reuters) - North Korean leader Kim Jong Un and Russian President Vladimir Putin will meet on Thursday in the Russian Pacific port of Vladivostok to discuss the international standoff over Pyongyang's nuclear programme, a Kremlin official said.
The visit is part of Kim's effort to build foreign support after the breakdown of a second U.S.-North Korea summit in Vietnam in February meant no relief on sanctions for North Korea, analysts said.
The summit will be the first between Putin and the North Korean leader and the nuclear row, and how to resolve it, would be the main item on the agenda, Kremlin aide Yuri Ushakov told reporters.
"In the last few months the situation around the peninsula has stabilised somewhat, thanks in large part to North Korea's initiatives of stopping rocket testing and closing its nuclear test site," Ushakov said. "Russia intends to help in any way possible to cement that positive trend."
The North's Korean Central News Agency (KCNA) said on Tuesday the visit would happen soon, but did not elaborate on a time or location.
Kim's chief aide, Kim Chang Son, was seen in Vladivostok on Sunday, according to South Korea's Yonhap news agency.
Vladivostok is the closest major Russian city to the short stretch of border that Russia and North Korea share, and can be reached from the border via train, Kim's preferred mode of international transport.
Russia has for years been involved in efforts to persuade North Korea to give up its nuclear progamme. It was involved in so-called six party talks - along with the two Koreas, Japan, the United States and China - that were last held in 2009.
South Korea's foreign ministry said it understood the agenda would include Russia-North Korea relations, denuclearisation, and regional cooperation.
"Russia shares our viewpoints such as the achievement of complete denuclearisation on the Korean Peninsula and the settlement of permanent peace," foreign ministry spokesman Kim In-chul said in Seoul.
"I hope that the summit will be an opportunity that contributes to positive progress."
After the failed Hanoi summit with U.S. President Donald Trump, Kim is probably looking to prove that he is still sought after by world leaders, and that he has more options, said Artyom Lukin, a professor at Far Eastern Federal University.
Kim did not want to look too dependent on Washington, Beijing and Seoul, he said.
"As for Russia, the Putin-Kim summit will reaffirm Moscow's place as a major player on the Korean Peninsula. This meeting is important for Russian international prestige."
Putin previously held a summit in Russia with Kim Jong Un's father and predecessor as North Korean leader, Kim Jong Il, in 2002. Kim Jong Il visited Russia again in 2011, when he was hosted by Dmitry Medvedev, the Putin lieutenant who at the time was serving as Russian president.
Online media which monitor North Korea reported that the venue for the summit would be the Far Eastern Federal University, on an island that is connected to the mainland part of Vladivostok by a bridge.
The bridge was built in time for the 2012 Asia-Pacific Cooperation Summit, which took place on the same site that now houses the university.
At the university campus on Tuesday, the sports complex had been closed and workers were seen bringing in pieces of furniture, a Reuters TV crew at the site reported. A white tent had been erected next to the sports complex.
At the entrance to the campus, security guards were stopping vehicles as they drove in, and searching them. There were no signs of preparation at Vladivostok railway station.
But at Khasan, a Russian settlement where the train line crosses the border, the state flags of Russia and North Korea were fluttering from the station building on Tuesday. A set of mobile steps for alighting from a train was positioned on the platform edge. (Additional reporting by Joyce Lee and Josh Smith in SEOUL; Maxim Rodionov in MOSCOW; Writing by Christian Lowe Editing by Angus MacSwan)
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